For example, a solid polymer fuel cell includes an electrolyte electrode assembly and separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte (electrolyte membrane) interposed between the anode and the cathode. The electrolyte is a polymer ion exchange membrane (proton ion exchange membrane). Generally, when this type of the fuel cell is used, predetermined numbers of electrolyte electrode assemblies and separators are stacked together to form a fuel cell stack.
In the fuel cell stack, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy. An oxidizing gas (reactant gas) such as a gas chiefly containing oxygen (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the fuel cell, a fuel gas flow field (reactant gas flow field) is formed on a surface of the separator facing the anode for supplying the fuel gas to the anode. An oxygen-containing gas flow field (reactant gas flow field) is formed on a surface of the separator facing the cathode for supplying the oxygen-containing gas to the cathode. Further, this type of the fuel cell adopts an internal manifold in which fluid passages extend through the electrolyte electrode assembly and the separators in the stacking direction for allowing the oxygen-containing gas and the fuel gas as reactant gases to flow into, and flow out of the reactant gas flow fields.
For example, in a solid polymer electrolyte fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2001-266910, as shown in FIG. 13, at one end of a separator 1 of the fuel cell in a direction indicated by an arrow X, an oxygen-containing gas supply passage 2a for supplying an oxidizing gas such as an oxygen-containing gas, and a fuel gas supply passage 3a for supplying a fuel gas such as a hydrogen-containing gas are formed.
At the other end of the separator 1 in the direction indicated by the arrow X, an oxygen-containing gas discharge passage 2b for discharging the oxygen-containing gas, a fuel gas discharge passage 3b for discharging the fuel gas are formed. The separator 1 has an oxygen-containing gas flow field 4 on its surface 1a facing a cathode (not shown). For example, the oxygen-containing gas flow field 4 includes a plurality of grooves extending in the direction indicated by the arrow X. The oxygen-containing gas flow field 4 is connected to the oxygen-containing gas supply passage 2a and the oxygen-containing gas discharge passage 2b. 
However, since the oxygen-containing gas supply passage 2a is provided at a right side of one end the separator 1 in the direction indicated by the arrow X, when the oxygen-containing gas is supplied from oxygen-containing gas supply passage 2a vertically in the direction indicated by the arrow X toward the oxygen-containing gas flow field 4, in particular, it is difficult to sufficiently supply the oxygen-containing gas to an area near the fuel gas supply passage 3a (left side of one end in the direction indicated by the arrow X).
Therefore, the number of grooves (part of the oxygen-containing gas flow field 4) for virtually guiding the oxygen-containing gas from the oxygen-containing gas supply passage 2a to the electrode surface is limited. Thus, the pressure loss in the grooves of the oxygen-containing gas flow field is increased. Components such as a compressor for supplying the oxygen-containing gas are large. Consequently, it is not possible to reduce the size and the weight of the facility. It is not possible to supply the reactant gas to the electrode surface uniformly, and the power generation performance is low.
Further, the separator 1 does not have any fluid passages on the two opposite sides 1b in the direction indicated by the arrow Y perpendicular to the direction indicated by the arrow X. Thus, these sides 1b tend to be exposed to the external air. Consequently, water condensation occurs in the flow grooves near the sides 1b. Since the condensed water is retained, the power generation performance may be lowered undesirably.